Main house water shut off is kaput!

We just bought a house in the southern US built in 2002 that sat unoccupied for a whole year. It has a well. The water at all faucets was UGLY for a couple weeks but has mostly cleared up after 3 months. Every time we open the outdoor hydrant it gushes black for a couple seconds, then rust color, then clears. An upstairs bathtub runs rust colored for a couple seconds each time it is turned on. The master bathroom tap is always cloudy white no matter how long it runs and tastes chalky. The kitchen faucet has low pressure even with filter screen removed. So I decided we need a whole house water filter. Today I went around looking for the main shutoff and a place to install the filter. There are no valves outside except for the hydrant which is right next to the well casing. I see no water tank anywhere except for the 2 gas water heaters in the attic.
The apparent main shut off valve is in the attached garage with only the valve wheel protruding from the drywall. I say apparent shut off valve because it is unmarked and when I turned it clockwise it reduced the water flow in the kitchen by only half but the wheel kept turning for several revolutions and never became tight. When I turned it back counter clockwise it again kept turning and never reached a stop. Worse, the water pressure I lost by turning clockwise never came back.
My wife will be home in a few hours. Is there anything I can do immediately that will get it back the way it was to save me from a beating?
What is the right thing to do to fix the problems I mentioned long term? Is there a way to do it without breaking sheetrock? Filter reccomendations?